Process for case carburizing and heat treating metals



Aug. 4, 1931. 1,817,407

PROCESS FOR CASE CARBURIZING AND HEAT TREATING METALS P. v. BROWER' Filed July 19. 1927 INVENTOR:

U BY.

Qwsluw ATTORNEYS PatentedAug: 4, 1931.

UNITED STATES PATENT OFFICE.

PAUL V. BRbWEB, OF KENMORE, NEW YORK, ASSIGNOB '10 CARBIDE & CARBON CHEMICALS CORPORATION. A

CORPORATION OF NEW YORK ritocsssron CASE cannuarzme AND HEAT TREATIHG METALS .A'ppllcation filed m 19,

This invention relates: to the art of carburizing and heat treating metals, and more specifically to "an improved process and apparatus adapted for case carburizing or heat treating metal,-'particularly articles of steel and iron, by means of mixtures of water vapor and hydrocarbon gases.

To eliminate deposition of excess carbon within a carburizing chamber or'retort and 10 on the work therein, various hydrocarbon gases have heretofore been diluted with air, ammonia, and other gases to provide mix-. tures adapted for case ,carburizing metals. Because of its availability, air as a hydrocarbon diluent has advantages over other gases generally employed. However, its

.use requires rather expensive mixing apparatus and-even then it is difficult if not impossible to maintain a mixture of con- :3 stant proportions for widely different volumes delivered by the gas mixing apparatus. Moreover, in some instances the use of air as a diluent"isobjectionable because the oxygen thereoffsupports the combustion 'of some hydrocarbons to such a degree thatthe temperature inside the carburizing retort is increased "above the desired value, tending to overheat the steel or other article and thereby causing objectionable effects such as -warping, coarse brittle grain structures, and excessively high carbon content in the carburized zone of the metal. Air also produces an. explosive mixture with some proportions of certain gaseous hydrocarbons; and in some instancesis also objectionable because it contains a high percentage of nitrogen. Where a good carbide case is desired, it is impracticable to use ammonia as a diluent because a combined nitride and carbide case is obtained. The corrosive action of am monia on various parts of the equipment is a decided draw-back; leaky apparatus leads hydrocarbon is passed to mix the latter with to shut-downs for repairs especially because 1927. Serial No. 206,881.

ammonia, add practical obstacles to its use in gascarbur z1ng.-

Accordingly, the principal object of this invention is to provide a gaseous mixture that shall be adapted for carburizing and heat treating metals, free from the numerous objections just mentioned. Another object of this inventionis to provide a gas car- I burizing process and apparatus in which mixture pressures above atmospheric, i. e. pressure carburizing, may be employed without complicated as well as expensive mixing appartus and pressure blowers.

- The above and otherobjects and the novel features of this invention will be apparent from the following description taken with the accompanying drawing which diagrammatically illustrates an apparatus adapted for case carburizing and heat treating according to the present invention.

Broadly speaking, my improved process involves the use of a mixture of water vapor and a suitable gaseous hydrocarbon. The hydrocarbon gas may be propane, propylene, butane, butylene, iso-butane, ethane, ethylene, methane, city gas, natural gas, etc., and mixtures of. some of these gases. Such gases as propane, propylene, butane, butylene, iso-butane and certain mixtures thereof are preferred here because they are readily liquefiable at to say, at temperatures of about 70 Fl and pressures under 200 pounds per square inch, so that large volumes thereof may be conveniently stored in tank cars and portable cylinders; and, inasmuch as the vapor pressures of such liquefied gases are substantial-' 1y constant, for a given purity and tempera ture the pressure of the gas delivered through the usual pressure reducing valve will remain practicallyconstant for all volumes of liquefied gas in the container. I also prefer to use the readily liquefiable gases just mentioned because t eir uniform carbon content gives uniform and dependable results as compared with city'gases and natural gases heretofore extensively used. The use of such ases or mixtures, undiluted, for carburizing iron and steel is accompanied by a decidedly objectionable deordinary temperatures, that is ion posit of excess carbon within the retort or reaction chamber, such excess carbon in time so clogging valves and piping leading to and from the carburizing chamber as to prevent the flow of the carburizing gas. Moreover, such a deposit is difiicult to separate from the work, generally requiring additional operations such as brushing or treatment in a revolving cylinder with sand and water, and subsequent drying by tumbling in sawdust. Hence, the expense of using undiluted gaseous h drocarbons is excessive and it is difficult if not impossible to obtain adequate penetration into the surface of the iron or steel. pletel removed, also" contaminates the queue ing medium and clogs parts of the circulating system thereof.

In my process I prefer to employ a readily liquefiable gas such as propane, commercially supplied in cylinders and tank cars in a liquidstate at pressures determined by ,the vapor pressure of propane, which varies from about 25 pounds at 0 F. to approximately 110 pounds per square inch at F., and. higher at higher temperatures. The liquefied propane is preferably vaporized within the cylinder, then as a gas passes through a well known type of pressure reducingand regulating valve and is delivered at the desired pressure beneath the surface of a body of water that is substantially free from dissolved gas other than propane or equivalent hydrocarbon gas. The water is maintained at a predetermined temperature dependent upon the pressure of the hydrocarbon gas and the propane bubbling therethrough takes up a predetermined amount proper proportions.

of water vapor 'to form a mixture of the D This mixture is then applied directly to the work, in the present instance being delivered directly into the carburizing retort and from the latter through an outlet valve to the atmosphere, when vitiated.

The accompanying drawing diagrammatically illustrates-the application of the inventlon to a number of carburizing retorts, the same prlnciples being applicable where a larger or smaller number of retortsof the same or different type are employed. As

shown, the propane in a liquid state and under pressure may be contained in cylinder C from which it is delivered through a suitable pressure reducing and regulating valve V of well known construction. From the valve V the propane is delivered as'a gas at the desired pressure through a pipe P to one or more branch pipes P which have outlets well below the surface of heated water in a corresponding number of chambers W. In bubbling through the water in the chambers W, the propane takes up a predetermined amount of water vapor, the

" mixture of "water vapor and hydrocarbon The carbon deposit, when not comgas being delivered through outlet pipes W to suitable means for utilizing the same, e. g., the carburizing retorts R of the furnaces F. Such retorts and furnaces inay be of a Well known or any preferred type, being constructed to receive the articles to be case hardened and provided with gas inlets and outlets and appliances, as required. An example of a suitable retort and furnace is disclosed in Patent No. 822,460 issued June .5, 1906. The main gas supply line P and each of the branch pipes P are provided with valveswhereby the supply of propane may be shut off from or opened to all or any one of the carburizing retorts, as desired.

In order to produce a carburizing mixture having the proper proportions of propane and Water vapor, it is essential to maintain the water in the several chambers W at a substantially constant elevated temperature. The water in the chambers W may be individually or collectively maintained at the required temperature in a number of ways. In the apparatusillustrated, I prefer to employ a single Water supply and heating means comprising a tank'H and a heating coil H, water being maintained at the same level in the tank H and in the chambers W by a float L that controls an inlet valve L communicating with a water supply pipe S. The water in the system maybe automatically maintained at the required temperature by a thermostatically controlled heating means, such as a gas burner G which heats the coil-H and has its main gas supply controlled by a well known type of thermostat valve T operatively associated with the water in the tank H. The gas for the burner G is preferably though not necessarily supplied through pipe G from the same cylinder that supplies the propane to be mixed with water vapor, and a suitable by-pass G" may be provided around the main valve to supply sufiicient gas for a pilot light at the burner G.

means for mixing the propane or other hydrocarbon gas with water vapor may be employed. For example, water or steam at an appropriate temperature and hydrocarbon gas may be simultaneously delivered to an injector of well known construction,

about one pound above atmospheric pressure shalltake up an equal volume of water vapor I. have found that it. is necessary to maintain the temperature of the water in thechambers' YV between 180 and 190 \Vhere the pressure of the hydrocarbon gas is varied, as determined by the desired carbon content of the carburized zone and .the desired speed of penetration of the carbon in forming the case, the water temperature is "simultaneously varied. This simplification is of considerable practical importance,

for when higher pressures of propane are employed, it is merely necessary to raise the temperature of the water in order to insure that enough water vapor is taken up by the gaseous propane to prevent carbon deposition.

Therefore the present process very readily lends itself to pressure carburizing, which is commercially desirable because it increases the rate of penetration of the carbon into steel by as much as 80% in instances where pressures of from about 15 to 535. pounds above atmospheric are used. Gas carburization at such pressures has heretofore been found to be impracticable because (1) the use of such gases as city gas and natural gas often produces an excess percentage of carbon content in the carburized zone, even at atmospheric pressure, and of'higher carbon content and deeper penetration at higher pressures, causing brittleness in the outer part of the carburized zone; and (2) gas compression for this purpose to pressures above 6 to 8 pounds becomes increasingly expensive. With the improved mixture of water vapor and propane at a carburizing pressure one pound above atmospheric I am able 'to obtain a case having a carbon content of about 0.95 to 1.00% in the outer part of the carburized zone; and carburized zones higher in carbon than about 1.2% produced by employing this mixture at pressures above atmospheric, are not deeper than the amount of metal that is usually removed in finishing the.metal surface, thus leaving a case hardened surface on the finished article having not greater than about 1.2% carbon. With gases heretofore used under similar conditions the carbon content of the outer zone is consider While the action of my improved mixture in producing these new results is not exactly known to me, and therefore cannot be stated as a fact, the improvement in the control of the carbon content of the case is probably due to the formation of carbon monoxide.

when the water of the mixture combines at carburizing temperatures with the excess carbon of the. propane, according to the following equation:

The hydrogen so formed probably also combines with excess carbon of the propane to produce simpler hydrocarbons, such as methane, which are carburizing agents. The carbon monoxide, being a rather slow carburizer and particularly one that gives a low carbon content, c. g., 0.6% in the outer part of the carburized zone, coasting with the remaining propane, and reduces the carbon concentration that would be obtained with propane alone. penetration, using my improved mixture, is not reduced but, on the contrary, is slightly increased.

Another advantage of my process arises from the fact that I am able to carburize at temperatures as low'as 1550 F. and at pressures of upwards of 15 pounds per square inch gage and yet obtain a rate of penetration substantially the same as that obtained when employing considerably higher carburizing temperatures. It is well known that low temperature carburization is highly desirable because distortion of the work is less apt to occur, and grain growth is greatly reduced, which in some instances eliminates heat treating operations often required to refine the grain structure of the core.

On the other hand, I may employ my improved mixture to carburize at higher temperatures up to 1850 F. and at pressures satisfactory case carburization of the depth desired. The apparatus operates effectively for all ranges of gas flow through the water, i. e., from zero velocity to a reasonably fast velocity, and at any requiredvelocity toinsure a proper supply of uniform mixture for any particular design of furnace. By

simply shutting off the valve in any branch propane supply pipe or the valve in the main-propane supply .pipe the gas and vapor mixing operation in any or all of the'chambers W may be interrupted as desired. This is particularly advantageous since it provides a simple means of stop-' Moreover, the rate of ping the operation in any furnace and eliminates mistakes which are apt to occur where a number of "manipulations are required to interrupt such operation, as when other methods of producing the mixture are, employed. For illustrative purposes and with no intention of limiting the in.- vention thereto, I give below a specific example of my improved process and the results, obtained therefrom. The work, such as a number of 0.2% carbon steel articles, was charged in the usual manner into a carburizing chamber, which was heated to about 1700 F. A substantially half-andhalf mixture of water vapor and propane, produced by bubbling the propane under a pressure of about 1 pound above atmospheric through water otherwise free from dissolved gas and at a temperature of about 186 F., was passed through the chamber for about three hours while the chamber was held at1700 F. At the end of this run, the depth of the case was about 0.037 inches. The outer 0.013 inches of the case had a carbon content of 0.05 to 1.00% next came a eutectoid region of 0.85 to 0.90% carbon having a depth of about 0.0088 inches; next a zone of a depth of about 0.010 inches having 0.65 to 0.70% carbon; and finally a zone of a depth of about 0.005 inches having 0.65 to 0.20% carbon. The subsequent heat treatment of these articles was according to well known S. A. E. specifications.

Mixtures of water vapor and hydrocarbons of the class described may be used advantageously in heat treating operations, such as annealing; and such operations may be performed with the herein-described apparatus. Generally speaking, .the proportions of the constituents of a particular m1x-,

ture required depends upon various factors, for example, the composition/of the steel or iron being treated. Highcarbon steels require an atmosphere high in carbon content, whereas low carbon steels should be annealed in an atmosphere of relatively low stances the hydrocarbon gas, e. g. propane,

is present in such proportions as to produce xa substantiafly neutral atmosphere to prevent decarburization, oxidization or other changes in the steel undergoing annealing, while in some instances the mixture may have moderate carburizing properties. Sometimes the mixture is circulated slowly around the steel while it is being heated,

The temperaupon various factors, such as the composition of the steel and the results desired from the operation, and may be as high as about 1700 F.

I claim: I

1. Process of case carburizing a steel or iron article, comprising a plying to the surface of such article while the latter is maintained at a carburizin mixture of water vapor an a gas comprising a gaseous hydrocarbon, the volume of said gaseous hydrocarbon comprising substantially 50% of said mixture.

2. Process of case carburizing a steel or iron article, comprising applylng to the surface of such article while the latter is maintained at a carburizin temperature a mixture of water vapor an a gas comprising propane,said mixture comprising substantially equal parts of water vapor and propane.

3. Process of case carburizing a steel or iron article, comprisingtreating the article heated to a carburizing temperature with a gaseous mixture comprising a substantial amount each of water vapor and propane.

In testimony whereof, I afiix my signature PAUL V. BROWER.

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